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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

ALLOYS OF GOLD  AND COPPER

287

for finenesses between 350 and 500, the 750 acid for finesses 500-750, and
the goo acid for finenesses above 750.    The compositions of these acids are :

50O acid.
Nitric acid (D 1-384), 40

c.c.
Hydrochloric    acid    (D

1-19), 0-5 c.c.
Distilled water, 20 c.c.

750 acid.
Nitric acid (D i'346), 98

c.c.

Hydrochloric    acid

1-171), 2 c.c.
Distilled water, 25 c.c.

(D

900 acid.
Nitric acid (D 1-384), 40

c.c.
Hydrochloric    acid    (D

1-19), 5 c.c.
Distilled water, 15 c.c.

The procedure consists in nibbing the sample on the stone so as to leave
a sharp line 3-4 mm. wide and 15-20 mm. long, and quite close to this,
two streaks with two needles one less and the other more fine than the
external characters of the alloy would indicate for it. The streaks are
observed in the light and compared. A little of the acid corresponding
with the fineness of the comparison needles is then rubbed with a glass
rod over the streaks, which are then compared, dried with absorbent paper,
again treated with acid, again dried, and the residues of gold on the stone
compared. From the colour of the streaks before treatment with acid
and the intensity of those remaining after the action of the acid, the approxi-
mate fineness of the alloy is judged. If the streak left by the sample is
not comparable with those of the needles, the test must be repeated with
needles of higher or lower fineness.

ACTUAL TEST. Apparatus and reagents, (i) A muffle furnace, cupels,
and a thermo-electric couple like that used in the cupellation of silver (q.v.}.

(2)  Silver of 999 fineness absolutely free from gold, and lead which need
not be free from silver.

(3)  Highly resistant pear-shaped flasks with stout, very long necks (assay
flasks) and a crucible of very fine refractoiy earth or of unglazed porcelain
or graphite.

Amounts of silver and lead to be used in the cupellation. The assay of
the gold is carried out in duplicate on 0-5 gram of the sample, and the amount
of silver to be added for the inquartation is about three times (more exactly
2-5 times) that of the gold. Thus, if the touchstone assay indicates an
approximate fineness of 900, the amount of silver to be added to each o-5
gram of the alloy is 0-9 X 2-5 X 0-5 = 1-125 gram. The silver, which is
weighed to the nearest centigram, should be in sheet which is not too thin,
so that the piece used in each case forms two squares of 0-5 cm. side.

The amount of lead required is also related to the fineness of the alloy
and is given by the following table :

Amount of lead (grams) required
per 0-5 gram of sample.

Fineness of
the sample.

IOOO
QOO
800
700
600
500
400100

8-0
n-o

I2-O
13-0

Procedure.   The gold is sampled in the manner indicated for silver (q.v.},esence of bismuth or lead, which may be identified